Expert Advisors

In the block of accounting orders the number and quality of orders existing in a client terminal for a security (to the window of which the EA is attached) is detected. In this block orders of other securities must be eliminated. If a programmed trading strategy requires using only market orders (and does not use pending orders) the fact of presence of pending orders must be detected. If a strategy admits only one market order and there are actually several orders, this fact should also be known. The task of the order accounting block (in this scheme) is in defining whether the current trading situation corresponds with an expected one, i.e. that in which the EA can adequately operate. If the situation corresponds, control must be passed to the next block to continue the EA's operation; if not, the EA's operation must be terminated and this fact must be reported to a user.
If there are no orders in the terminal or the number and quality of existing orders corresponds to what was expected, control is passed to the block of defining trading criteria. In this block all criteria necessary for making trade decisions are calculated, namely criteria for opening, closing and modifying orders. Further control is passed to the block of closing orders.
It is easy to understand why in the offered scheme the block of closing orders is executed earlier than the block of opening orders. It is always more reasonable to process first existing orders (close or modify) and only after that to open new orders. Generally, it is correct to be guided by the desire to have as little orders as possible. During the execution of this block all orders, for which the closing criterion has been activated, must be closed.
After all necessary orders has been closed, control is passed to a block of new orders size calculation. There are a lot of algorithms for calculating an order volume. The simplest of them is using a constant, fixed lot size. It is convenient to use this algorithm in a program for testing strategies. More popular method of defining an order size is setting the number of lots depending on the amount of free margin, for example 30-40% of it. If free margin is not enough, the program terminates its operation having informed a user about the reason.
After the number of lots for opening new orders is defined, control is passed to order opening block. If any of criteria calculated earlier points to the necessity of opening an order of a certain type, a trade request to open an order is formed in this block.
There is also error analyzing block in an Expert Advisor. If any trade operation failed, control (only in this case) is passed to the error processing block. If an error returned by a server or client terminal is not crucial, one more attempt is made to perform a trade operation. If a crucial error is returned (for example, an account is blocked), an EA must terminate its operation. Remember, in MQL4 there is no possibility of program terminating an EA's operation in a security window (as distinct from scripts, see Special Functions). What can be done in a program way is the termination of start(). At a new start of the function start() on a new tick the value of a certain variable-flag prohibiting trading (in this case enabled as a result of a critical error) can be analyzed and control can be passed for the termination of the special function operation; thus formation of new trade request is not permitted. In the offered scheme the flag value is analysed in the block of preliminary processing.


Trading Strategy




Market prices are constantly moving. Market state at any moment of time can be conditionally characterized either as a trend - strong unidirectional price change (rise or fall), or as a flat - lateral price movement with weak deviations from a certain average. These market characteristics are conditional, because there are no clear criteria, according to which trend or flat can be identified. For example, long lateral movements with strong deviations that can be traced neither to a flat nor to a trend. Generally it is assumed that the market is mainly in the state of lateral movement and trends usually take place 15-20% of time.


All trading strategies also can be conventionally divided into two main groups. The first group contains flat-oriented strategies. The main idea of such strategies is that after an evident deviation price must return to the previous position, that's why orders are opened in the direction contrary to the last price movement. The second group strategies are trend strategies, when orders are opened in the same direction as the salt price movement. There are more complicated (combined) strategies. Such strategies take into account many different factors that characterize market; as a result trading can be executed both on flat and trend. It is not hard to implement trading according to this or that strategy technically - MQL4 contains all necessary means for it. The main work in the creation of once own strategy consists in the search of trading criteria.


Trading Criteria




In this example we will try to construct a trend Expert Advisor, i.e. the one that will open orders in the price movement direction. So, we need to find among various technical indicators those that detect a trend beginning. One of the simplest methods of searching trading criteria is based on the analysis of the combination of MAs with different averaging periods. Fig. 111 and Fig. 112 show the position of two different MA (with periods of averaging 11 and 31) on different market parts. Averages with small averaging period (red lines) are closer to a price chart, twisty and movable. Moving averages with larger period of averaging (blue line) are more inert, have larger lag and are situated farther from market prices. Let's pay attention to places where MAs with different averaging periods cross and try to decide, whether the fact of MA crossing can be used as a reading criterion.


Number of Orders




In this example we analyze an Expert Advisor that admits presence of only one market order, pending orders are not provided. Such an approach is justified not only in this certain example, but can be used as the basis for any strategy.
Pending orders are usually used when a developer has quite a reliable criterion for forecasting the future price change with high probability. If there is no such criterion, no need to use pending orders.
The situation when several opposite orders for one security are open also cannot be considered reasonable. It was written earlier that from economical point of view opposite orders are considered to be senseless, especially if the order prices are equal (see Closing and Deleting Orders). In such a case we should close one order by another one and wait for a signal to open one market order in a certain direction.






SOURCE : http://articles.mql4.com/84